Heterojunction bipolar transistors have recently been used as transistors for power amplifier modules in mobile terminal devices and other devices. This type of bipolar transistor is called an HBT (Hetero Junction Bipolar Transistor).
A bipolar transistor disclosed in Min-Chang Tu, Herng-Yih Ueng, and Yu-Chi Wang: “Performance of High-Reliability and High-Linearity InGaP/GaAs HBT PAs for Wireless Communication” IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 1, JANUARY (2010) p 188 is described as a first example of the prior art of such a bipolar transistor. As shown in FIG. 30, in the bipolar transistor, a subcollector layer 102 is formed in contact with a semiconductor substrate 101, such as that formed of GaAs, and a collector layer 103 is formed in contact with the subcollector layer 102. A base layer 104 is formed in contact with the collector layer 103, and an emitter layer 105 is formed in contact with the base layer 104. An emitter electrode 111 is formed in contact with the emitter layer 105. A base electrode 110 is formed in contact with the base layer 104. A collector electrode 109 is formed in contact with the subcollector layer 102.
In the bipolar transistor according to the first example of the prior art, the emitter layer 105 and the base layer 104 form a heterojunction. The emitter layer 105 is designed to have a wider band gap than the base layer 104. Furthermore, the collector layer 103 is formed to have a uniform impurity concentration in the thickness direction (depth direction).
Next, a bipolar transistor disclosed in Japanese Unexamined Patent Application Publication No. 02-291135 is described as a second example of the prior art of a bipolar transistor. As shown in FIG. 31, in the bipolar transistor according to the second example of the prior art, particularly the collector layer 103 includes a first collector layer 103a, a second collector layer 103b, and a third collector layer 103c. The first collector layer 103a having a relatively low impurity concentration and the second collector layer 103b having a relatively high impurity concentration form a junction. Since the other components are the same as in the bipolar transistor according to the first example of the prior art, the same components are denoted by the same reference numerals and description thereof is not repeated.